Styrene removal in paraxylene recovery process

ABSTRACT

The invention relates to removal of styrene from hydrocarbon mixtures, and more particularly, removal of styrene from hydrocarbon mixtures containing higher than equilibrium paraxylene concentrations.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority to and the benefit of U.S. ProvisionalApplication No. 61/681,486, filed on Aug. 9, 2012, and U.S. ProvisionalApplication No. 61/653,688, filed on May 31, 2012, the disclosures ofwhich are incorporated herein by reference in their entirety.

FIELD OF THE INVENTION

The invention relates to purification of process streams in theproduction of paraxylene by alkylation of benzene and/or toluene and tothe removal of styrene from aromatic hydrocarbon streams containinghigher than equilibrium concentrations of paraxylene.

BACKGROUND OF THE INVENTION

Of the aromatic C8 isomers, including the three xylene isomers andethylbenzene, paraxylene is of particularly high value since paraxyleneis useful in the manufacture of synthetic fibers and resins. Refineryand chemical plant streams containing the aromatic C8 isomers typicallycontain, at thermodynamic equilibrium, only about 22-24 wt % paraxylene,based on the weight of the xylene isomers in the stream. Separation ofparaxylene from the other C8 isomers requires superfractionation and/ormultistage refrigeration steps and/or adsorptive separation, all ofwhich are energy intensive. There is a need to provide processes forproducing paraxylene in more efficient ways, such as in higherselectivity than can be obtained from refinery and chemical plantstreams.

One known method for producing paraxylene selectively involves thealkylation of toluene and/or benzene with methanol and/or DME(dimethylether) over a solid acid catalyst. Selectivities to paraxylenein excess of 90 wt % (based on total C8 aromatic product) have beenreported by reacting toluene with methanol in the presence of a catalystcomprising a porous crystalline material, preferably a medium-porezeolite and particularly ZSM-5, having a Diffusion Parameter for2,2-dimethylbutane of about 0.1-15 sec⁻¹ when measured at a temperatureof 120° C. and a 2,2-dimethylbutane pressure of 60 torr (8 kPa). SeeU.S. Pat. Nos. 6,423,879 and 6,504,072.

WO 99/38823 reported a reactive distillation process comprising thecontact of toluene with a methylating agent in the presence of a zeolitein a reaction/distillation column produces, as a side product, DME,which can be recycled (with unreacted methanol) to extinction in theprocess. The process operates at no greater than 320° C.

It has recently been discovered that the alkylation of benzene and/ortoluene with methanol can also result in the production of a variety ofoxygenates, in addition to DME, but also other oxygenate by-products.See for instance U.S. patent application Ser. No. 13/487,651. Accordingto the invention described in Ser. No. 13/487,651, the concentration ofphenolic impurities in a xylene stream produced by alkylation of benzeneand/or toluene with methanol can be reduced to trace levels, e.g., below0.1 ppmw, by one or more washing treatments with an aqueous solution ofa base. The resultant treated xylene stream, if necessary after waterwashing to remove any phenate-containing solution, can then be recycledto the xylene splitter to generate additional para-xylene or can be usedas a solvent.

Recently, a process for the production of paraxylene selectively by: (i)reacting of toluene and/or benzene with methanol in the presence of asuitable catalyst under appropriate conditions to process streamcomprising paraxylene in higher than equilibrium amounts; (ii) contactof said process stream comprising paraxylene in higher than equilibriumamounts with a suitable adsorbent to remove phenol, said phenol havingbeen produced in (i), or is present in the feedstream of toluene and/orbenzene and/or alkylating agent (methanol and/or DME), or anycombination thereof, to provide a product stream having a lowerconcentration of phenol than said process stream, has been described inU.S. Provisional Patent Application No. 61/653,698.

It has also recently been discovered that xylenes produced by alkylatingtoluene and/or benzene with an alkylating agent comprising methanoland/or DME over a solid acid catalyst contain small quantities ofstyrene, which, if not removed, could cause operability problems fordownstream paraxylene recovery processes, or even further, in processesusing paraxylene, such as the production of terephthalic acid, andderivatives thereof, including polyester fibers, films, and the like.

Several characteristics of the xylene produced in this manner makestyrene removal challenging. The desired product, paraxylene, is presentat higher-than-equilibrium concentration. The catalyst used to removestyrene must therefore show minimal xylenes isomerization activity. Thecatalyst must also minimize formation of benzene, which also can havedetrimental effects on downstream processing. Furthermore, as alreadymentioned, the product contains a variety of oxygenates, such as phenol.Moreover, olefinic compounds may enter the alkylation reaction systemvia the feedstream of toluene such as catalytic reforming units, whichare a source of toluene for the aforementioned alkylation reaction.These and other problems make the treatment of the product stream fromthe alkylation of benzene and/or toluene in the presence of an acidcatalyst difficult.

U.S. Pat. No. 4,795,550 teaches converting trace quantities of olefinicimpurities to nonolefinic hydrocarbons by contacting an aromatic processstream from an alkylation reaction with a solid catalyst compositecomprising a crystalline aluminosilicate zeolite and a refractoryinorganic oxide. Faujasite is mentioned as a preferred aluminosilicatezeolite and the refractory inorganic oxide can be alumina,silica-alumina, or a mixture of both.

U.S. Pat. No. 6,313,362 teaches that in an alkylation/transalkylationprocess, an aromatic alkylation process stream comprising polyalkylatedaromatic compounds is contacted with a purification medium in a liquidphase pre-reaction step, prior to transalkylation, to remove impurities,including styrene. A large pore molecular sieve catalyst such as MCM-22may be used as the purification medium in the pre-reaction step becauseof its high reactivity for alkylation, strong retention of catalystpoison and low reactivity for oligomerization under the pre-reactorconditions. The alkylation processes envisioned include the productionof ethylbenzene, cumene, ethyltoluene, and cymenes.

U.S. Pat. No. 7,731,839 teaches treating an aromatic hydrocarbonfeedstock having undesirable olefins including styrene with a catalystsuch as MCM-22 to reduce the amount of undesirable olefins. Likewise,U.S. Pat. No. 6,005,156 teaches a process for the reduction of olefinsand diolefins from mixtures of aromatic hydrocarbon-rich cuts bytreatment in a hydrogenation zone and then treatment with clay. Althoughmany sources of aromatic-rich hydrocarbon cuts are mentioned, a processfor alkylation of benzene and/or toluene with methanol is notrecognized. Similarly, U.S. Pat. No. 7,199,275 teaches treatment of apartially dehydrated aromatic feedstock containing styrene as animpurity by contacting with a first molecular sieve having a Si/Al molarratio less than about 5 and a second molecular sieve having a Si/Almolar ratio of greater than about 5. The thus-treated feedstock is thenused for an alkylation reaction of benzene with ethylene and/orpropylene or transalkylation reactions in the liquid phase.

Other references include FR 2295935, teaching the reduction of olefinand diene content of an aromatics-rich fraction by subjecting thefraction to an acid-catalyzed vapor or liquid-phase alkylation reaction;JP 2138137A, teaching separation of styrene from a C8 aromatics streamby selective adsorption of styrene with a modified faujasite zeolite;and JP 76026421B, teaching isolating styrene from a hydrocarbon fractioncomprising styrene and ethylbenzene and/or xylene isomers by adsorptionof styrene with a zeolite comprising an alkali or alkaline earth metal.

As far as the present inventors are aware, the prior art has notaddressed the problem of styrene impurities in a system for alkylationof benzene and/or toluene with an alkylating agent selected frommethanol, DME, and mixtures thereof, in the presence of a catalyst toproduce a process stream comprising higher than equilibrium amounts ofparaxylene and with the co-production of styrene.

The present inventors have surprisingly discovered a method of purifyingsaid process stream of styrene impurities without significant loss ofthe desired paraxylene product or co-production of additionalimpurities.

SUMMARY OF THE INVENTION

The invention is directed to the purification of an aromatic hydrocarbonstream including selective removal of styrene from a process stream,said process comprising the contact of benzene and/or toluene with analkylating agent, in the presence of a suitable alkylation catalystunder appropriate conditions to selectively produce paraxylene, saidselective removal of styrene comprising the contact of said processstream with a suitable material under conditions effective to provide aproduct stream from said process, said product stream having a lowerconcentration of styrene than said process stream, preferably less than30 ppm, more preferably less than 20 ppm. The alkylating agent ispreferably selected from methanol, dimethylether (DME) and mixturesthereof.

The process stream treated to provide a lower concentration of styrenemay be the feedstream to the process, such as the toluene stream from acatalytic reforming unit, or a stream comprising xylenes downstream ofthe alkylation reactor, such as upstream of a fractionator used toseparate unreacted toluene (e.g., “detol fractionator”) and/or methanolfrom the alkylation reactor product xylene stream, the bottoms and/oroverhead product from said detol fractionator, a xylenes splitter, suchas utilized to separate heavy aromatics (C9+ aromatic hydrocarbons) fromthe xylene product, upstream or downstream from a paraxylene recoveryunit (e.g., adsorptive separation, such as a Parex™ adsorptiveseparation unit or Eluxyl™ adsorptive separation unit, or acrystallization apparatus), upstream or downstream of an isomerizationunit (which may be a liquid phase or vapor phase isomerization unit, inseries or parallel), and the like. The process stream may also comprisean imported process stream or any other type of stream which has pickedup styrenic impurities, particularly styrene such as from a previouscargo.

The process is also directed to a process for the production ofparaxylene selectively by: (i) reacting of toluene and/or benzene withmethanol and/or DME in the presence of a suitable alkylation catalystunder appropriate conditions to produce a process stream comprisingparaxylene in higher than equilibrium amounts and styrene; (ii) contactof said process stream comprising paraxylene in higher than equilibriumamounts and styrene with a suitable material to remove at least some ofsaid styrene and provide a product stream having lower concentration ofstyrene than said process stream.

By “selectively produce paraxylene” is meant the production of xyleneswherein paraxylene is present in amounts greater than is present in anequilibrium mixture of C8 aromatic isomers, and by “equilibrium mixture”or “equilibrium amounts” with reference to the concentration ofparaxylene in a mixture of C8 aromatic isomers is meant generally about22-24 wt %. Preferably the alkylation reaction produces a product streamhaving at least 70 wt %, such as 75 wt %, 80 wt %, 85 wt %, 90 wt %,(all wt % herein based on the total amount of C8 aromatic isomers unlessotherwise specified), to about 99 wt % or even higher, particularly inthe ranges of 70-90 wt %, 75-88 wt %, 80-95 wt %, 82-88 wt %, or in therange of from any lower wt % disclosed to any higher wt % disclosed.

In embodiments the amount of styrene present after said contact with acatalyst suitable for selective removal of styrene is less than 20 ppmwt, more preferably less than 10 ppm wt. based on the total amount ofaromatic hydrocarbon.

By “selective removal of styrene” means that the amount of styreneremoved in the styrene-removal contacting step is, in embodiments, equalto or greater than the amount of styrene produced in the alkylationreaction, or so that the final product after the styrene removal step isless than 20 ppm, or less than 10 ppm, and/or greater than the amount ofbenzene produced in said step, and/or greater than the amount ofparaxylene isomerization that occurs in said styrene removal step.

In embodiments said process stream may be subjected to additionalprocess steps such as fractionation, adsorptive separations,crystallization, membrane separation, and the like, to remove speciesother than phenol.

In embodiments, said contact may be in the presence of hydrogen or itcan be in the absence of hydrogen.

In embodiment said contact occurs in the temperature range of 100 to275° C., more preferably 103 to 180° C.

In embodiments the production of benzene in said styrene removal step isin amounts of 30 ppm wt. or less based on the total amount of aromatichydrocarbon, and/or the amount of paraxylene isomerization is less than1 wt %, based on the amount of paraxylene.

In embodiments the material used in the styrene removal step is selectedfrom MWW molecular sieves, clay, and mixtures thereof, such as at leastone of MCM-22, MCM-36, MCM-49, MCM-56, EMM-10 molecular sieves, andEngelhard F-24, Filtrol 24, Filtrol 25, and Filtrol 62 clays, Attapulgusclay and Tonsil clay. The molecular sieves have been described innumerous patents and publications, such as U.S. Pat. No. 4,954,325; U.S.Pat. No. 5,229,341; U.S. Pat. No. 5,236,575; and U.S. Pat. No.5,362,697, and the clays are likewise well-known. Any of these arecommercially available.

It is an object of the invention to provide a continuous,semi-continuous, or batch process of purifying xylene feedstream ofstyrene impurities with minimal co-production of benzene and minimalisomerization of paraxylene to another C8 aromatic isomer.

It is another object of the invention to provide an apparatus adaptedfor the process of the invention.

These and other objects, features, and advantages will become apparentas reference is made to the following detailed description, preferredembodiments, examples, and appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 and 2 are schematic diagrams showing certain preferredembodiments of the present invention.

FIGS. 3-6 show experimental results on one or more embodiments of theinvention.

DETAILED DESCRIPTION

According to the invention, xylenes produced by alkylating tolueneand/or benzene with methanol and/or dimethylether (DME) with a catalystsuch as a solid acid catalyst contain small quantities of styrene,which, if not removed, could cause operability problems for downstreampara-xylene recovery processes. The invention is a method of removingthe styrene by contact with an appropriate material under conditionssufficient to reduce the amount of styrene without production ofsignificant amount benzene and/or isomerization of paraxylene, and alsoto apparatus adapted for said process.

Without wishing to be bound by theory, in embodiments, it is believedthat styrene is converting to a heavy aromatic product (e.g., C9aromatics) over said catalyst under appropriate conditions to reduce theamount of styrene without production of significant amounts of benzeneor isomerization of paraxylene. The heavy product could then be removedin downstream distillation.

The invention may also be practiced using a feedstream of benzene,toluene, or any combination thereof, as the aromatic species to bealkylated, and a feedstream of methanol, dimethylether (DME), and anycombination thereof, as the alkylating agent.

A typical composition of xylenes produced from the reaction of toluenewith methanol in the presence of a solid acid catalyst is shown in Table1.

TABLE 1 Compound wt % Toluene 0.0950 Styrene 0.0648 mixed xylenes 94.56Ethylbenzene 0.47 C9 aromatics 4.7574 phenol 0.0014 Other oxygenates>0.05

Representative species found in the product include1,2,4-trimethylbenzene, 1-methyl-4-ethylbenzene, n-nonane, naphthalene,1,4-dimethyl-2-ethylbenzene, biphenyl, anthracene, C16 aromaticalkylates, benzoic acid, 4-methylbenzoic acid, o-cresol,2,4-dimethylphenol, and the like. The presence of styrene wouldtypically not be expected in a stream of paraxylene-enriched xylenes(that is, paraxylene in higher than equilibrium amounts).

The invention may be better understood by reference to FIG. 1, which isa schematic illustration of preferred embodiments of the invention. Oneof skill in the art in possession of the present discourse willunderstand that the invention may be practiced other than asspecifically illustrated in FIG. 1, and the illustration is not intendedto be limiting.

In FIG. 1, apparatus 1 is a reactor suitable for carrying out alkylationof benzene and/or toluene in the presence of a solid acid catalyst, suchas ZSM-5, particularly a phosphorus-containing ZSM-5, and moreparticularly a phosphorus-containing ZSM-5 that has been steamed attemperatures on the order of 1000° F. The specifics of the apparatus 1do not form a part of the present invention, except as otherwise statedherein, but rather have been described in numerous prior art patents,patent application, and publications.

The effluent from 1, which is an aromatic hydrocarbon stream rich inparaxylene, such as 70 wt % or more, as described in more detail herein,a typical composition of which is shown in Table 1, above, is passed viaconduit 11 to styrene removal unit 2A. The unit 2A is illustrated withdotted lines since it is but one possible placement of such a unit,albeit preferred, according to embodiments of the invention. Thestyrene-depleted stream rich in paraxylene is passed via conduit 12 to afractionation column or splitter 3 wherein the stream is split into aparaxylene-enriched overhead passed downstream in the process throughconduit 13 and a paraxylene depleted bottoms product passed downstreamin the process through conduit 14.

Styrene removal unit 2B illustrates a second possible placement of astyrene removal unit according to the present invention. One or both of2A and 2B may be used in an embodiment of the process according to theinvention. Styrene removal units may comprise one or more vesselscontaining, by way of example, MCM-22. Such units are per se known inthe art.

The paraxylene-enriched overhead from splitter 3, is passed via conduit15 to an optional second fractionation column 7, or in the case whereunit 2A is not present, a first styrene removal unit 2B. Theparaxylene-enriched stream 15, now having been treated by one or both ofstyrene removal units 2A and/or 2B is then passed via conduit 15 tofractionator 7 to remove heavies such as aromatic C9+s 21 and pass anoverhead 16, comprising a paraxylene-enriched, styrene-depleted stream(relative to effluent 11), further downstream in the process. Thisstream 16 may optionally be treated in apparatus 4 to remove otherimpurities, such as oxygenates, e.g., by an absorbent-based phenoladsorption and/or such as by a caustic wash. See, for instance, U.S.Provisional Patent Application No. 61/653,698 and U.S. patentapplication Ser. No. 13/487,651. The stream 17 is then sent toparaxylene removal unit 5 of the type known in the art, such as anadsorptive separation unit (e.g., Parex unit or Eluxyl unit) orseparation by crystallization. The final desired high purity paraxylenestream is taken off via conduit 18 and the paraxylene-depleted stream22, otherwise known as raffinate with respect to paraxylene removalunits, is sent to a liquid phase isomerization unit 6, of the typewell-known in the art, wherein the paraxylene-depleted raffinate isisomerized to equilibrium concentration xylenes (i.e., about 22-24 wt %paraxylene). The isomerization may be liquid phase or vapor phase, bothper se known in the art. After isomerization the equilibrium xylenesstream may be recycled via line 20 to fractionator 7 or otherwiseprocessed as desired.

The invention may be still be even better understood by reference toFIG. 2, which is a schematic illustration of another preferredembodiment of the invention. One of skill in the art in possession ofthe present discourse will understand that the invention may bepracticed other than as specifically illustrated in either FIG. 1 or 2,and the illustrations is not intended to be limiting.

FIG. 2 illustrates apparatus downstream of the alkylation reactor, notshown for convenience of view. In FIG. 2, fresh feed 100, comprisingalkylating agents selected from methanol, DME, or a combination thereof,and/or the aromatic species to be alkylated, selected from benzene,toluene, and mixtures thereof, are passed through phenol-removal unit101 and then via conduit 102 to styrene-removal unit 103, and thenthrough conduit 104 to fractionator 110, which removes toluene overheadto be sent to the alkylation reactor, as described further hereinbelow.One of the advantages of the present invention is that one or more ofextracted toluene and non-extracted toluene may be used, and sincenumerous sources of toluene can be used in the alkylation reactor usinga solid acid catalyst, as well as numerous sources of methanol and/orDME alkylating agent. Thus, it is advantageous to have a pre-treatmentof the feed(s) in one or both of units 101 and 103 as set forth in FIG.2, however use of such pretreatment is optional. It will also beappreciated that the alkylating agent(s) and benzene and/or toluene canbe fed separately together, and likewise the choice of whether to have apretreatment by one or both of units 101 and 103, in either order, canbe made independently on each separate feedstream. Water isadvantageously also added to the upstream alkylation reactor and may beadded into the system represented by the entirety of the alkylationreactor (not shown) and the apparatus shown in FIG. 2 along with feed100, or it may be added separately to said system directly to thealkylation reactor; the addition of water and the location of additionis optional. Advantageously, the addition of water into the alkylationreactor is in an amount sufficient to reduce coking of the catalyst. Oneor more of the feeds may be added preheated and the feed(s) may be addedas liquid or vapor to reactor 100.

Continuing with FIG. 2, 105 represents one or more of the recycledreactor products, advantageously previously treated to remove gaseousproducts (e.g., light olefins) and possibly dried of water, although, asmentioned above, water may be added as part of the feed. The removal ofgaseous products and/or water is not shown in FIG. 2 as it does not forma part of the present invention per se, and could be accomplished by oneof ordinary skill in the art in possession of the present disclosure.Typically unreacted methanol and/or DME and unreacted benzene and/ortoluene are recycled via line 105 or they may be recycled separately, ora combination thereof, and thus pretreatment by phenol-removal unit 106and/or styrene removal unit 108, fluidly connected to 106 via line 107and to reactor 110 via line 109, is advantageous.

Continuing with FIG. 2, fractionator tower 110 (or “detol tower”)removes toluene overhead and the toluene can optionally be sent viaconduit 111 through one or more of phenol removal unit 112 and styreneremoval unit 114, fluidly connected by conduit 113, prior to beingrecycled to the alkylation reactor (again, not shown) via conduit 115.It will be appreciated that the toluene stream 111 may be optionallycooled to a liquid by known methods, not shown, prior to treatment inunits 112 and/or 114, and that whether or not to have a phenol removalstep and/or styrene removal step between the detol tower 110 and thealkylation reactor, and the order thereof, can be determinedindependently by one of ordinary skill in the art in possession of thepresent disclosure.

The bottoms product of detol tower 110, comprising the desiredparaxylene-enriched alkylation reactor products, leaves 110 throughconduit 116 and may optionally be treated by one or more of the phenolremoval unit 117 and/or the styrene removal unit 119, fluidly connectedby conduit 118, and is then passed to xylene splitter 121 via conduit120 to remove heavier products (i.e., C9+ aromatic hydrocarbons) as abottoms product through conduit 122. The C9+s may be advantageously usedwith a transalkylation unit integrated with the system shown in FIG. 2,and indeed the entire apparatus shown in FIG. 2, and the alkylationreactor not shown, may be advantageous integrated with other knownmethods of generating xylenes and/or other aromatic products, forgreater efficiency of feedstream use, integration of heat (which inembodiments is generated by the alkylation reactor), and so on.

The overheads 123 from xylene splitter 121 may also optionally betreated by one or both of phenol removal unit 124 and styrene removalunit 126, fluidly connected via conduit 125 prior to being sent viaconduit 127 to an optional second xylene splitter 128, which analogousto splitter 121, separates the desired paraxylene-enriched xylenesstream overhead, to conduit 130, from bottoms product 129, which mayagain be disposed of in the same manner as bottoms product 122.

The overheads 130 may be treated by one or both of a phenol removal unitor styrene removal unit, and in the embodiment of FIG. 2, is illustratedby a single unit 131, which may represent one or more of such units.

The remaining portions of FIG. 2 are similar to the per se well-knownxylenes loop, but integrated with the styrene removal step(s) accordingto the present invention, and thus, the paraxylene-enriched stream 132is passed to a paraxylene recovery unit 133, which may be an adsorptionunit (e.g., Parex adsorption unit or Eluxyl adsorption unit), or acrystallizer unit, or membrane unit, and the like, wherein paraxylene isseparated from its C8 aromatic isomers, to yield very high purityparaxylene via conduit 134, which may yet further be treated by one ormore of a phenol-removal unit and/or styrene removal unit, representedby a single apparatus 135, to yield a highly pure, highly enrichedparaxylene product, having, in embodiments, a paraxylene content ofgreater than 99.0 wt % (based on the entire content of the stream), andfurther characterized by one or more of: (i) a styrene content of lessthan 30 wppm; and (ii) a phenol content of less than 1 wppm.

Finally, continuing with the description of the embodiment shown in FIG.2, the raffinate (paraxylene-depleted xylenes stream) from theparaxylene removal unit 133, leaving via conduit 137, may be recoveredas mixed xylenes 139, optionally but advantageously treated by one ormore of a styrene removal unit and/or a phenol removal unit, againrepresented by a single apparatus 138, and/or via conduit 140 to theisomerization unit represented by apparatus 143, optionally butadvantageously treated by one or more of a styrene removal unit and/or aphenol removal unit, again represented by a single apparatus 141 whichis fluidly connected to isomerization unit 143 by conduit 142. Theproduct of the isomerization unit, as is well-known, will be anequilibrium stream of xylenes, which may be advantageously then recycledback xylenes splitter 128 and/or sent to ethylbenzene purge (not shown)via conduit 145, optionally treated by one or more of phenol removaland/or styrene removal unit, again represented by a single unit 146,leaving said unit, if present, via conduit 147.

The isomerization unit 143 may be liquid phase or gas phase or both maybe used in series or parallel. Both liquid and gas phase isomerizationprocesses and units to use in said processes are per se well-known inthe art.

Material that can be used for styrene removal according to the presentinvention, for example in one or more units 2A and/or 2B such asillustrated schematically in FIG. 1, or one or more of units 103, 108,115, 119, 126 and also 131, 135, 138, 141, and 146, such as illustratedschematically in FIG. 2, include members of the MWW family of zeolites(MCM-22, MCM-49, MCM-56, etc.), and clays. These zeolites can beproduced in various formulations, including those bound with clay,alumina, or silica, and self-bound formulations. Fresh or regeneratedcatalysts can be used, as well as catalyst that has been reprocessedafter use in a different service, such as ethylbenzene or cumeneproduction. Another type of catalyst that could be used for styreneremoval from these xylene streams are clays. Catalyst beds that containmixed layer(s) of clay and MWW family zeolite catalysts could also beused. MWW and methods of making it, molecular formula, and other methodsof characterization, are per se well-known in the art; see for instance,U.S. Pat. No. 5,001,295.

In embodiments, the process for styrene removal from xylenes streamsproduced by the reaction of methanol and benzene and/or toluene bycontact with the appropriate catalyst can use fixed-bed, adiabaticreactors operating at temperatures such as from 100-300° C. and WHSVsuch as from 1.0-100 hr-1, preferably with pressure high enough tomaintain liquid-phase conditions.

Phenol removal, which is the subject of U.S. Provisional PatentApplication No. 61/653,698 and more fully explained therein, may beaccomplished by numerous means, including the use of fixed bedadsorbents such as alumina, silica, ion-exchange resins, and zeolites.Particularly advantageous adsorbents are those which may be regenerated,such as by one or more techniques including (1) purge with hot N₂ suchas at elevated temperatures such as >150° C.; (2) purge with mixture(s)of N₂ and at least one organic solvent such as aromatics, alcohols,ketones, etc. or at least one inorganic solvent such as water, CO₂, CS₂,etc., at temperatures such as >20° C.; (3) purge with an organic solventsuch as aromatics, alcohols, ketones, etc., or an inorganic solvent suchas water, CO₂, CS2, and the like, in either liquid phase or vapor phaseat temperatures such as >20° C. followed by a N₂ purge at elevatedtemperatures such as >100° C.; (4) purge with mixture(s) of at least oneorganic solvent and at least one inorganic solvent at temperatures issuch as >25° C. followed by a N₂ purge at elevated temperatures suchas >150° C.; or (5) purge with air, mixture of oxygen and nitrogen,steam, or mixture thereof at elevated temperatures such as >150° C.

In order to more fully understand the present invention the followingdetailed experiments are described. It will be understood that theexperiments are not intended to be limiting but that the invention canbe practiced otherwise than specifically described.

Experimental Run 1

A reactor containing 65% MCM-22/35% alumina binder catalyst was fed axylenes stream comprising 650 ppm (wt) styrene and no heavies (A9+s) oroxygenates at 9.8 WHSV and 265 psig (1827 kPa). No hydrogen was present.Analysis of the outlet stream (such as by gas chromatography (GC)) showshigh styrene conversion can be achieved at temperatures of 103-180° C.(less than 20 ppm styrene by wt, generally less than 10 ppm by wt, andoften less than 5 ppm by wt). Benzene formation was below 70 ppm over aperiod of 760 hours. Negligible xylenes isomerization is seen at theseconditions.

Experimental Run 2

The run above was continued (i.e., same reactor and catalyst) but thefeed was switched to a xylenes feed containing heavies and oxygenateswhen the reactor temperature in the run above reached 275° C. The newfeed, with heavies and oxygenates, accelerated catalyst deactivation.Similar styrene conversion was maintained by raising the reactortemperature and/or addition of hydrogen, such as 15 ppm wt. H₂. Thereaction was stopped when the temperature reached 275° C. and the outletstyrene concentration went above about 45 ppm wt.

Table 2 compares the results at the end of Experiment Run 1 and thestart of the subsequent Experiment Run 2. In the second run, there wassignificant xylenes isomerization and benzene formation over MCM-22 at275° C. at SOR (start of run). Whereas in the first run, noisomerization and acceptable benzene make were seen with de-edgedcatalyst near EOR (end of run). “TOS” is time on stream. WHSV is weighthourly space velocity. Part per million are in weight (wppm).

TABLE 2 Styrene Removal Run# 1 2 TOS (hr) 1923 68 Catalyst 65% MCM-2265% MCM-22 Reactor Temperature (deg C.) 275 275 WHSV (hr⁻¹) 9.8 9.8 FeedH2 (ppm) 15 15 Product Styrene (ppm) 38 0.42 Product Benzene (ppm) 26303 Product PX Selectivity (%) 79.1 56.2

A low-reactor temperature startup can achieve adequate styreneconversion while avoiding excess xylenes isomerization and benzeneyield. The temperature can be raised and/or hydrogen introduced as thecatalyst ages to maintain styrene conversion while continuing to avoidexcess xylenes isomerization and benzene make.

Additional examples, again not intended to be limiting, follow. Theseexamples are representative of the removal of styrene from xylenesproduct of the alkylation process over a 65% MCM-22/35% aluminacatalyst. Similar results would be expected for other olefinic compoundsin other non-olefinic hydrocarbon streams over other solid acidcatalysts under conditions other than specifically set forth herein.

FIGS. 3, 4, and 5 show results from a laboratory experiment using a 65%MCM-22/35% alumina binder catalyst running at 2.5 WHSV and 265 psigusing a xylenes feed with a greater than equilibrium (nominally 79%)concentration of para-xylene that was spiked with 650 ppm styrene.During the initial few days of operation, the reactor temperature wasvaried over a range of 180-275° C.

The effect of temperature on the effluent styrene concentration, theeffluent benzene concentration, and paraxylene loss due to isomerizationare shown in FIGS. 3, 4, and 5, respectively. The effluent styreneconcentration was maintained at a consistent <1 wppm level over theentire temperature range, while the effluent benzene concentration andthe paraxylene loss increased substantially as the temperature wasincreased. This shows that controlling the bed temperature can reducethe undesirable side reactions while maintaining good removalefficiency.

Table 3 shows results at two different WHSVs from a laboratoryexperiment using a 65% MCM-22/35% alumina binder catalyst running at abed temperature of 225° C. and 265 psig using a xylenes feed with agreater than equilibrium (nominally 79%) concentration of paraxylenethat was spiked with 650 wppm styrene. The reactor was at 9.8 WHSV andthe flow rate was reduced to a WHSV of 2.5. The effluent styreneconcentration dropped dramatically with the reduction in flow rate butthe effluent benzene concentration increased only modestly and theparaxylene loss was unchanged. This shows that controlling the bed flowrate can maintain good removal efficiency without increasing theundesirable side reactions.

TABLE 3 WHSV 9.8 2.5 Effluent Styrene Concentration, wppm 50.7 4.1Effluent Benzene Concentration, wppm 0.0 7.4 Para-xylene loss, % 0.5 0.5

FIG. 6 shows results from another laboratory experiment using a 65%MCM-22/35% alumina binder catalyst running at 9.8 WHSV and 265 psigusing a xylenes feed with a greater than equilibrium (nominally 79%)concentration of para-xylene that was spiked with 650 ppm styrene. Theeffluent styrene concentration is plotted in FIG. 6 as a function of thecumulative BI-bbl converterted/lb catalyst (BI/bbl is “BromineIndex/barrel”, both being well-known per se). The reactor temperature inthis laboratory experiment was initially at 180° C. and was increasedwhenever the effluent styrene approached or exceeded the target value of20 wppm. These results show that the styrene conversion can bemaintained within specification for a time by raising the reactortemperature to offset the loss of activity as the catalyst ages.

The alkylation process employed herein can employ any aromatic feedstockcomprising toluene and/or benzene, although in general it is preferredthat the aromatic feed contains at least 90 wt %, especially at least 99wt %, of benzene, toluene or a mixture thereof. An aromatic feedcontaining at least 99 wt % toluene is particularly desirable.Similarly, although the composition of the methanol-containing feed isnot critical, it is generally desirable to employ feeds containing atleast 90 wt %, especially at least 99 wt %, of methanol.

The catalyst employed in the alkylation process is generally a porouscrystalline material and, in one preferred embodiment, is a porouscrystalline material having a Diffusion Parameter for 2,2-dimethylbutaneof about 0.1-15 sec-1 when measured at a temperature of 120° C. and a2,2-dimethylbutane pressure of 60 torr (8 kPa).

As used herein, the Diffusion Parameter of a particular porouscrystalline material is defined as D/r²×106, wherein D is the diffusioncoefficient (cm2/sec) and r is the crystal radius (cm). The diffusionparameter can be derived from sorption measurements provided theassumption is made that the plane sheet model describes the diffusionprocess. Thus for a given sorbate loading Q, the value Q/Qeq, where Qeqis the equilibrium sorbate loading, is mathematically related to(Dt/r²)^(1/2) where t is the time (sec) required to reach the sorbateloading Q. Graphical solutions for the plane sheet model are given by J.Crank in “The Mathematics of Diffusion”, Oxford University Press, ElyHouse, London, 1967.

The porous crystalline material is preferably a medium-pore sizealuminosilicate zeolite. Medium pore zeolites are generally defined asthose having a pore size of about 5 to about 7 Angstroms, such that thezeolite freely sorbs molecules such as n-hexane, 3-methylpentane,benzene and p-xylene. Another common definition for medium pore zeolitesinvolves the Constraint Index test which is described in U.S. Pat. No.4,016,218, which is incorporated herein by reference. In this case,medium pore zeolites have a Constraint Index of about 1-12, as measuredon the zeolite alone without the introduction of oxide modifiers andprior to any steaming to adjust the diffusivity of the catalyst. Inaddition to the medium-pore size aluminosilicate zeolites, other mediumpore acidic metallosilicates, such as silicoaluminophosphates (SAPOs),can be used in the present process.

Particular examples of suitable medium pore zeolites include ZSM-5,ZSM-11, ZSM-12, ZSM-22, ZSM-23, ZSM-35, and ZSM-48, with ZSM-5 andZSM-11 being particularly preferred. In one embodiment, the zeoliteemployed in the process of the invention is ZSM-5 having a silica toalumina molar ratio of at least 250, as measured prior to any treatmentof the zeolite to adjust its diffusivity.

Zeolite ZSM-5 and the conventional preparation thereof are described inU.S. Pat. No. 3,702,886. Zeolite ZSM-11 and the conventional preparationthereof, are described in U.S. Pat. No. 3,709,979. Zeolite ZSM-12 andthe conventional preparation thereof, are described in U.S. Pat. No.3,832,449. Zeolite ZSM-23 and the conventional preparation thereof, aredescribed in U.S. Pat. No. 4,076,842. Zeolite ZSM-35 and theconventional preparation thereof, are described in U.S. Pat. No.4,016,245. ZSM-48 and the conventional preparation thereof, are taughtby U.S. Pat. No. 4,375,573. The entire disclosures of these U.S. patentsare incorporated herein by reference.

The medium pore zeolites described above are preferred for the presentprocess since the size and shape of their pores favor the production ofp-xylene over the other xylene isomers. However, conventional forms ofthese zeolites have Diffusion Parameter values in excess of the 0.1-15sec-1 range desired for the present process. Nevertheless, the requireddiffusivity can be achieved by severely steaming the zeolite so as toeffect a controlled reduction in the micropore volume of the catalyst tonot less than 50%, and preferably 50-90%, of that of the unsteamedcatalyst. Reduction in micropore volume is monitored by measuring then-hexane adsorption capacity of the zeolite, before and after steaming,at 90° C. and 75 torr n-hexane pressure.

Steaming to achieve the desired reduction in the micropore volume of theporous crystalline material can be effected by heating the material inthe presence of steam at a temperature of at least about 950° C.,preferably about 950 to about 1075° C., and most preferably about 1000to about 1050° C. for about 10 minutes to about 10 hours, preferablyfrom 30 minutes to 5 hours.

To effect the desired controlled reduction in diffusivity and microporevolume, it may be desirable to combine the porous crystalline material,prior to steaming, with at least one oxide modifier, preferably selectedfrom oxides of the elements of Groups IIA, IIIA, IIIB, IVA, VA, VB andVIA of the Periodic Table (IUPAC version). Conveniently, said at leastone oxide modifier is selected from oxides of boron, magnesium, calcium,lanthanum and preferably phosphorus. In some cases, it may be desirableto combine the porous crystalline material with more than one oxidemodifier, for example a combination of phosphorus with calcium and/ormagnesium, since in this way it may be possible to reduce the steamingseverity needed to achieve a target diffusivity value. The total amountof oxide modifier present in the catalyst, as measured on an elementalbasis, may be between about 0.05 and about 20 wt %, such as betweenabout 0.1 and about 10 wt %, based on the weight of the final catalyst.

Where the modifier includes phosphorus, incorporation of modifier in thealkylation catalyst is conveniently achieved by the methods described inU.S. Pat. Nos. 4,356,338; 5,110,776; 5,231,064; and 5,348,643; theentire disclosures of which are incorporated herein by reference.Treatment with phosphorus-containing compounds can readily beaccomplished by contacting the porous crystalline material, either aloneor in combination with a binder or matrix material, with a solution ofan appropriate phosphorus compound, followed by drying and calcining toconvert the phosphorus to its oxide form. Contact with thephosphorus-containing compound is generally conducted at a temperatureof about 25° C. and about 125° C. for a time between about 15 minutesand about 20 hours. The concentration of the phosphorus in the contactmixture may be between about 0.01 and about 30 wt %.

Representative phosphorus-containing compounds which may be used toincorporate a phosphorus oxide modifier into the catalyst of theinvention include derivatives of groups represented by PX3, RPX2, R2PX,R3P, X3PO, (XO)3PO, (XO)3P, R3P═O, R3P═S, RPO2, RPS2, RP(O)(OX)2,RP(S)(SX)2, R2P(O)OX, R2P(S)SX, RP(OX)2, RP(SX)2, ROP(OX)2, RSP(SX)2,(RS)2PSP(SR)2, and (RO)2POP(OR)2, where R is an alkyl or aryl, such asphenyl radical, and X is hydrogen, R, or halide. These compounds includeprimary, RPH2, secondary, R2PH, and tertiary, R3P, phosphines such asbutyl phosphine, the tertiary phosphine oxides, R3PO, such as tributylphosphine oxide, the tertiary phosphine sulfides, R3PS, the primary,RP(O)(OX)2, and secondary, R2P(O)OX, phosphonic acids such as benzenephosphonic acid, the corresponding sulfur derivatives such as RP(S)(SX)2and R2P(S)SX, the esters of the phosphonic acids such as dialkylphosphonate, (RO)2P(O)H, dialkyl alkyl phosphonates, (RO)2P(O)R, andalkyl dialkylphosphinates, (RO)P(O)R2; phosphinous acids, R2PDX, such asdiethylphosphinous acid, primary, (RO)P(OX)2, secondary, (RO)2PDX, andtertiary, (RO)3P, phosphites, and esters thereof such as the monopropylester, alkyl dialkylphosphinites, (RO)PR2, and dialkyl alkyphosphinite,(RO)2PR, esters. Corresponding sulfur derivatives may also be employedincluding (RS)2P(S)H, (RS)2P(S)R, (RS)P(S)R2, R2PSX, (RS)P(SX)2,(RS)2PSX, (RS)3P, (RS)PR2, and (RS)2PR. Examples of phosphite estersinclude trimethylphosphite, triethylphosphite, diisopropylphosphite,butylphosphite, and pyrophosphites such as tetraethylpyrophosphite. Thealkyl groups in the mentioned compounds preferably contain one to fourcarbon atoms.

Other suitable phosphorus-containing compounds include ammonium hydrogenphosphate, the phosphorus halides such as phosphorus trichloride,bromide, and iodide, alkyl phosphorodichloridites, (RO)PCl2,dialkylphosphoro-chloridites, (RO)2PCl, dialkylphosphinochloridites,R2PCl, alkyl alkylphosphonochloridates, (RO)(R)P(O)Cl, dialkylphosphinochloridates, R2P(O)Cl, and RP(O)Cl2. Applicable correspondingsulfur derivatives include (RS)PCl2, (RS)2PCl, (RS)(R)P(S)Cl, andR2P(S)Cl.

Particular phosphorus-containing compounds include ammonium phosphate,ammonium dihydrogen phosphate, diammonium hydrogen phosphate, diphenylphosphine chloride, trimethylphosphite, phosphorus trichloride,phosphoric acid, phenyl phosphine oxychloride, trimethylphosphate,diphenyl phosphinous acid, diphenyl phosphinic acid,diethylchlorothiophosphate, methyl acid phosphate, and otheralcohol-P2O5 reaction products.

Representative boron-containing compounds which may be used toincorporate a boron oxide modifier into the catalyst of the inventioninclude boric acid, trimethylborate, boron oxide, boron sulfide, boronhydride, butylboron dimethoxide, butylboric acid, dimethylboricanhydride, hexamethylborazine, phenyl boric acid, triethylborane,diborane and triphenyl boron.

Representative magnesium-containing compounds include magnesium acetate,magnesium nitrate, magnesium benzoate, magnesium propionate, magnesium2-ethylhexoate, magnesium carbonate, magnesium formate, magnesiumoxylate, magnesium bromide, magnesium hydride, magnesium lactate,magnesium laurate, magnesium oleate, magnesium palmitate, magnesiumsalicylate, magnesium stearate and magnesium sulfide.

Representative calcium-containing compounds include calcium acetate,calcium acetylacetonate, calcium carbonate, calcium chloride, calciummethoxide, calcium naphthenate, calcium nitrate, calcium phosphate,calcium stearate and calcium sulfate.

Representative lanthanum-containing compounds include lanthanum acetate,lanthanum acetylacetonate, lanthanum carbonate, lanthanum chloride,lanthanum hydroxide, lanthanum nitrate, lanthanum phosphate andlanthanum sulfate.

The porous crystalline material employed in the process of the inventionmay be combined with a variety of binder or matrix materials resistantto the temperatures and other conditions employed in the process. Suchmaterials include active and inactive materials such as clays, silicaand/or metal oxides such as alumina. The latter may be either naturallyoccurring or in the form of gelatinous precipitates or gels includingmixtures of silica and metal oxides. Use of a material which is active,tends to change the conversion and/or selectivity of the catalyst andhence is generally not preferred. Inactive materials suitably serve asdiluents to control the amount of conversion in a given process so thatproducts can be obtained economically and orderly without employingother means for controlling the rate of reaction. These materials may beincorporated into naturally occurring clays, e.g., bentonite and kaolin,to improve the crush strength of the catalyst under commercial operatingconditions. Said materials, i.e., clays, oxides, etc., function asbinders for the catalyst. It is desirable to provide a catalyst havinggood crush strength because in commercial use it is desirable to preventthe catalyst from breaking down into powder-like materials. These clayand/or oxide binders have been employed normally only for the purpose ofimproving the crush strength of the catalyst.

Naturally occurring clays which can be composited with the porouscrystalline material include the montmorillonite and kaolin family,which families include the subbentonites, and the kaolins commonly knownas Dixie, McNamee, Georgia and Florida clays or others in which the mainmineral constituent is halloysite, kaolinite, dickite, nacrite, oranauxite. Such clays can be used in the raw state as originally mined orinitially subjected calcination, acid treatment or chemicalmodification.

In addition to the foregoing materials, the porous crystalline materialcan be composited with a porous matrix material such as silica-alumina,silica-magnesia, silica-zirconia, silica-thoria, silica-beryllia,silica-titania as well as ternary compositions such assilica-alumina-thoria, silica-alumina-zirconia, silica-alumina-magnesia,and silica-magnesia-zirconia.

The relative proportions of porous crystalline material and inorganicoxide matrix vary widely, with the content of the former ranging fromabout 1 to about 90% by weight and more usually, particularly when thecomposite is prepared in the form of beads, in the range of about 2 toabout 80 wt % of the composite.

The alkylation process can be conducted in any known reaction vessel butgenerally the methanol and aromatic feeds are contacted with thecatalyst described above with the catalyst particles being disposed inone or more fluidized beds. Each of the methanol and aromatic feeds canbe injected into the fluidized catalyst in a single stage. However, inone embodiment, the methanol feed is injected in stages into thefluidized catalyst at one or more locations downstream from the locationof the injection of the aromatic reactant into the fluidized catalyst.For example, the aromatic feed can be injected into a lower portion of asingle vertical fluidized bed of catalyst, with the methanol beinginjected into the bed at a plurality of vertically spaced intermediateportions of the bed and the product being removed from the top of thebed. Alternatively, the catalyst can be disposed in a plurality ofvertically spaced catalyst beds, with the aromatic feed being injectedinto a lower portion of the first fluidized bed and part of the methanolbeing injected into an intermediate portion of the first bed and part ofthe methanol being injected into or between adjacent downstream catalystbeds.

The conditions employed in the alkylation stage of the present processare not narrowly constrained but, in the case of the methylation oftoluene, generally include the following ranges: (a) temperature betweenabout 500 and about 700° C., such as between about 500 and about 600°C.; (b) pressure of between about 1 atmosphere and about 1000 psig(between about 100 and about 7000 kPa), such as between about 10 psigand about 200 psig (between about 170 and about 1480 kPa); (c) molestoluene/moles methanol (in the reactor charge) of at least about 0.2,such as from about 0.2 to about 20; and (d) a weight hourly spacevelocity (“WHSV”) for total hydrocarbon feed to the reactor(s) of about0.2 to about 1000, such as about 0.5 to about 500 for the aromaticreactant, and about 0.01 to about 100 for the combined methanol reagentstage flows, based on total catalyst in the reactor(s).

The product of the reaction between the methanol and the aromatic feedis a gaseous effluent comprising para-xylene and other xylene isomers,water vapor, unreacted toluene and/or benzene, unreacted methanol,phenolic impurities, light olefins and other light gas by-products, andgenerally some C9+ aromatic by-products. In addition, where the processis conducted in a fluidized catalyst bed, the effluent will contain someentrained solid catalyst and catalyst fines. Thus, the gaseous effluentleaving the (final) fluidized bed reactor is generally passed through anintegral cyclone separator to remove some of the entrained catalystsolids and return them to the alkylation reactor.

The product effluent leaves the alkylation reactor system at a hightemperature, typically between about 500 and about 600° C. and initiallymay be passed through a heat exchanger so that the waste heat in theeffluent stream may be recovered and used to heat other processstream(s). It is, however, preferred that any initial cooling of theproduct stream is limited so as to keep the effluent vapors well abovethe dew point, typically about 240° F. (116° C.).

Following further cooling, the effluent vapor stream is fed to aseparation system, which may comprise one or more fractionation columns,where the unreacted methanol and aromatics are recovered and recycled tothe alkylation step, the light and heavy hydrocarbons are removed andthe remainder of effluent is separated into a liquid organic phase richin xylene and a waste water stream. Part of the phenolic impurities areconcentrated in the xylene-rich organic phase and part are dissolved inthe waste water stream making the waste water stream acidic.

Typically, the phenolic impurities include phenol, methyl phenols anddimethyl phenols and are present in the xylene filtrate in an amountfrom about 0.2 ppmw to about 1000 ppmw of phenol, from about 0.2 ppmw toabout 1000 ppmw of methyl phenols and from about 0.5 ppmw to about 1000ppmw of dimethyl phenols.

The present invention can be integrated with other systems using tolueneand benzene streams, such as selective disproportionation of tolueneand/or transalkylation of toluene and aromatic C9+ species.

The invention has been described above with reference to numerousembodiments and specific examples. Many variations will suggestthemselves to those skilled in this art in light of the above detaileddescription.

Trade names used herein are indicated by a ™ symbol or ® symbol,indicating that the names may be protected by certain trademark rights,e.g., they may be registered trademarks in various jurisdictions. Allpatents and patent applications, test procedures (such as ASTM methods,UL methods, and the like), and other documents cited herein are fullyincorporated by reference to the extent such disclosure is notinconsistent with this invention and for all jurisdictions in which suchincorporation is permitted. When numerical lower limits and numericalupper limits are listed herein, ranges from any lower limit to any upperlimit are contemplated. While the illustrative embodiments of theinvention have been described with particularity, it will be understoodthat various other modifications will be apparent to and can be readilymade by those skilled in the art without departing from the spirit andscope of the invention. Accordingly, it is not intended that the scopeof the claims appended hereto be limited to the examples anddescriptions set forth herein but rather that the claims be construed asencompassing all the features of patentable novelty which reside in thepresent invention, including all features which would be treated asequivalents thereof by those skilled in the art to which the inventionpertains.

What is claimed is:
 1. A process for the purification of an aromatichydrocarbon stream including selective removal of styrene, comprisingcontact of said aromatic hydrocarbon process stream containing styreneand at least 70 wt % paraxylene, based on the total amount of C8aromatic species in said process stream, with at least one materialselected from MWW molecular sieves, clays, and mixtures thereof, toprovide a product stream having lower concentration of styrene than saidprocess stream, and wherein the selective removal of styrene causes lessthan 5 wt % of paraxylene to be lost to isomerization by said contact,wherein said contact comprises initially contacting said aromatichydrocarbon process stream with said at least one material at atemperature of about 100° C. to about 180° C. to effectively removestyrene until paraxylene lost by isomerization is less than 5 wt % andsubsequently raising said temperature to about 275° C. in a step-wisemanner while maintaining paraxylene lost by isomerization to less than 5wt %.
 2. The process according to claim 1, wherein said process streamcomprises effluent downstream of an alkylation process comprising thereaction of an alkylating agent selected from methanol, DME, andmixtures thereof, with benzene and/or toluene in the presence of asuitable alkylation catalyst under appropriate conditions to providesaid process stream.
 3. The process according to claim 1, wherein saidprocess includes the production of paraxylene selectively, andcomprises: (i) reacting of toluene and/or benzene with methanol and/orDME in the presence of a suitable catalyst under appropriate conditionsto provide an aromatic hydrocarbon process stream comprising paraxylenein higher than equilibrium amounts and styrene; (ii) contact of saidaromatic hydrocarbon process stream with a material selected from MWWmolecular sieves, clays, and mixtures thereof, to remove at least aportion of said styrene so as to provide a product stream having lowerconcentration of styrene than said process stream.
 4. The processaccording to claim 1, wherein the amount of styrene present after saidcontact is less than 20 ppm wt, based on the total amount of aromatichydrocarbon.
 5. The process according to claim 1, wherein the amount ofstyrene present after said contact is less than 10 ppm wt., based on thetotal amount of aromatic hydrocarbon.
 6. The process according to claim1, wherein said process stream is subjected to at least one additionalprocess selected from fractionation, crystallization, and adsorptiveseparations.
 7. The process according to claim 6, wherein saidadditional process comprises a process to remove oxygenate impurities.8. The process according to claim 1, wherein said process is in thepresence of hydrogen.
 9. The process according to claim 1, wherein saidprocess is in the absence of hydrogen.
 10. The process according toclaim 1, wherein said material is selected from MCM-22, MCM-49, andMCM-56 molecular sieves, acid-treated clays, and mixtures thereof. 11.The process according to claim 1, wherein said material comprises MCM-22molecular sieves and said contact occurs in the temperature range of 100to 275° C.
 12. The process according to claim 1, wherein the productionof benzene as a result of said contact is in amounts of 30 ppm wt. orless, based on the total amount of aromatic hydrocarbon.
 13. The processaccording to claim 1, wherein said contact with the at least onematerial selected from MWW molecular sieves, clays, and mixtures thereofisomerizes paraxylene less than 5 wt %.
 14. The process according toclaim 1, wherein the amount of paraxylene isomerization by said contactwith the at least one material selected from MWW molecular sieves,clays, and mixtures thereof is not detectable, based on gaschromatographic (GC) analysis.
 15. The process according to claim 1,including the reaction of an alkylating agent selected from methanol,DME, and mixtures thereof, with benzene and/or toluene in the presenceof a phosphorus-containing ZSM-5 catalyst that has been steamed, toproduce said aromatic hydrocarbon process stream.